This invention relates to methods and apparatus used in making electronic components. In particular, the invention relates to a method for screen printing paste solder onto a board and an apparatus therefor, a method for soldering an electronic component onto a board using screen printing and an apparatus therefor.
An electronic component (hereinafter also referred to as "chip" is conventionally soldered onto a board as shown in FIG. 1. More specifically, first a printing step 211 is executed to print creamed solder or paste solder onto a board by a screen printing apparatus. Then, a chip mounting step 212 is carried out to mount a chip or chips on the board by means of a chip mounting apparatus. This is followed by execution of a reflow step 213, wherein the paste solder is melted in a reflow oven to fix the chip or chips on the board.
Such soldering is conventionally practiced in an open air atmosphere. Unfortunately, solder balls are dissociated from lands on the board during the soldering in the reflow oven. [No] soldering under increased moisture conditions causes generation of a number of solder balls to become dissociated from the land of the board, leading to a failure in the board.
To reduce the cleaning step normally required after soldering a chip or chips onto a board, the flux ingredient contained in the past solder is desirably reduced. Unfortunately, reflow soldering is conventionally carried out in the open air atmosphere, and a reduction in flux content in the paste solder fails to permit the paste solder to exhibit satisfactory soldering characteristics.
Thus, the reflow step 213 is followed by a cleaning step 214, as shown in FIG. 1.
It is conventionally proposed to carry out reflow soldering in a closed nitrogen atmosphere for the purpose of preventing oxidation of the paste solder. Unfortunately, this requires a soldering system in a closed structure, leading to an increase in the cost of the system and deterioration in the efficiency of the system.
In view of the foregoing disadvantages of the conventional reflow soldering, the assignee proposed a reflow oven for carrying out reflow soldering in a dry air atmosphere, as disclosed in U.S. Pat. No. 5,526,978. The proposed reflow oven exhibits the advantages of minimizing the generation of solder balls eliminating the necessity of a soldering system in a closed structure, and attaining screen printing and reflow soldering on a large scale.
Nevertheless, in the art, past solder is printed on a board screen printing apparatus having an open air atmosphere and a chip or chips are mounted on the board in chip mounting apparatus having an open air atmosphere. Unfortunately, this causes either absorption of moisture by the flux ingredient of the paste solder or condensation or dewing of moisture on the applied paste solder during printing and chip mounting. This results in the increased generation of solder balls regardless of whether reflow soldering is conducted in a dry air atmosphere.
The inventors have found that when the reflow oven has a dry air atmosphere, both printing of the paste solder and chip mounting are desirably carried out in a dry air atmosphere. However, the prior art fails to accomplish such an operation.